The present invention relates to a runner and a water wheel, which exhibit excellent wear and corrosion resistance characteristics, and a method of manufacturing the same.
In recent years, a water wheel is more and more used under conditions where solid substances, such as sediment, is contained in a fluid. A runner, as driven under those conditions, is damaged by the combination of the wear due to impact with the sediment (referred to as "sedimental wear") and cavitation damage. Therefore, the portion to be damaged has been either lined with a resin, such as rubber, or spray-coated with a very hard material, such as a ceramics material. This technique is disclosed in Japanese Patent Laid-Open No. 47477/1991.
Generally speaking, a runner having a three-dimensional shape is frequently manufactured using a casting method. In a large-sized runner, on the other hand, there sometimes a method of assembling the runner by welding is used. In this case, stainless materials are welded into an assembly so as to enhance the mechanical strength of the runner. Specifically, a blade, a crown and a band or a shroud are separately formed of stainless materials, and the blade is welded between the crown and the band or the shroud to form the runner. At this time, the runner thus prepared is subjected, if necessary, to heat treatment so as to remove the residual stress established during the welding operation. This technique is disclosed in Japanese Patent Laid-Open No. 140465/1990.
In the case of sediment wear, on the other hand, the use of a variety of hard coatings has been investigated. For example, the bucket and needle chip of the Pelton wheel have been coated on their inner faces with a tungsten carbide surface film having a gradient composition by a vacuum plasma spray-coating method, as disclosed in Japanese Patent Laid-Open No. 88201/1994. On the other hand, the portion of the water wheel limited to the runner, which will be worn by sediment, is build-up welded with a Co alloy containing C, Si, Mn, Cr, Mo, Ni or Fe, as disclosed in Japanese Patent Laid-Open No. 75767/1990.
When the water machinery is to be used under conditions where sediment is contained in the fluid, as described above, the runner has to be coated with a very hard material, so that it may be prevented from being damaged by sediment wear and cavitation damage. Because of the three-dimensional shape, however, the runner is not easy to coat with the hard material.
The plating method can be used easily to coat an article having a complicated shape, but this method has the following problems when applied to the present invention. The Cr plating method is one of the most widely used electrolytic plating methods and can provide the highest surface film hardness in Vickers hardness (as will be expressed as "Hv") of about 1,000, of all the plated films. Because of the electrolytic plating, however, the electrolytic concentration is caused to vary due to the shape so as to make it difficult to have an even film thickness.
Because of the strain in the surface film, moreover, it is difficult with the plating method to form a thick film, i.e., a film having a sufficient thickness to protect against sediment wear and cavitation damage. On the other hand, Ni--P plating is a nonelectrolytic plating method so that it can form a surface film having an even thickness independently of the shape. As in the Cr plating method, however, with the Ni--P plating method it is difficult, due to the strain in the surface film, to form a thick film or a sufficient surface film to protect against sediment wear and cavitation damage. Moreover, both the Cr plating method and the Ni--P plating method are not practical for application to large-sized parts, such as a water wheel runner, from the point of view of the required facilities, because they require a step of dipping an article in a bath.
The hard coating film to be formed by the spray-coating method has a sufficient wear resistance to sediment wear and cavitation damage and can produce a sufficiently thick surface film easily. However, the use of the spray-coating method has the following problems. A proper distance is required between a spray-coating gun and the runner so as to form a satisfactory spray-coated surface film, and a relatively wide space is required due to the restriction provided by the size of the spray-coating gun. Therefore, it is difficult to spray coat a three-dimensional runner in a narrow space with a satisfactory surface film having sufficient hardness and adhesive strength.
The technique, as disclosed in the aforementioned Japanese Patent Laid-Open No. 88201/1994, is directed to a vacuum plasma spray-coating method which is applied to small-shaped parts having shapes facilitating a spray-coating treatment, that is members to be coated which can be accommodated in an evacuated container for the treatment. Therefore, the application of such a vacuum plasma spray-coating method to large-sized parts is difficult, and the application of the vacuum plasma spray-coating method to three-dimensional parts having complicated shapes is even more difficult. These problems have not been solved by the technique as disclosed in the aforementioned Japanese Patent Laid-Open No. 88201/1994.
On the other hand, the technique, as disclosed in the aforementioned Japanese Patent Laid-Open No. 75767/1990, is directed to a build-up welding treatment which is applied to a portion easy to be treated. Since the build-up welding is accompanied by a melting, it can form a reformed layer having a high adhesive strength, but a high calorie source is required to cause deformation by heat if treatment is applied to a large area. Therefore, the application of this technique to a water wheel runner is practically limited to a limited portion, and so it is difficult to treat the entire face of the runner, as required. Moreover, the application of plasma spray-coating to three-dimensional parts having complicated shapes is also more difficult, as in the technique disclosed in the aforementioned Japanese Patent Laid-Open No. 88201/1994. In addition, the Co alloy has a surface film hardness which is insufficient in the case of high sediment density so that a sufficient wear resistance cannot be achieved. These problems have not been solved by the technique disclosed in the aforementioned Japanese Patent Laid-Open No. 75767/1990.
As described above, it is difficult to coat a runner having a three-dimensional shape in a narrow space with a satisfactory surface film having a sufficient hardness and adhesive strength by using the spray-coating method. Specifically, when the spray-coating method is used, it is necessary to use the aforementioned manufacturing method using a welding operation for the assembly. However, when a surface film coating is applied by the spray-coating method to a runner manufactured using a welding operation, the following problems have been brought to light by detailed investigations.
First of all, the parts, spray-coated all over the surfaces thereof, can neither be welded nor provide a satisfactory welding strength. Moreover, the temperature of the parts will be raised in the vicinity of the welded portions by the welding heat so that a thermal strain is caused in the hard surface films generally having a smaller coefficient of thermal expansion than those of metals, thereby to cause a separation of the surface films or a lower adhesive strength. This makes it impossible to effectively coat the welded portions and their vicinities with the surface films.
However, the welded portions of the runner, i.e., the joints between the crown and the blade and between the shroud or band and the blade, have a relatively high flow speed and a high sedimental collision frequency so that they are liable to experience sediment wear and cavitation damage. These portions are damaged, if coated with no surface film, by sediment wear and cavitation damage thereby to cause the performance to drop or the lifetime to shorten.
On the other hand, if the crown and the blade, and the shroud or band and the blade, spray-coated with no film in the vicinity of the welded portions, are welded, and these individual welded portions are then spray-coated, after being welded, with the surface films, the following problems result, as has been brought to light by the detailed investigations.
In order to achieve a sufficient adhesive strength, the surfaces to be coated have to be properly roughened, before being coated with the spray-coated films, by a sandblasting treatment. In the case of the present example, however, the spray-coated films are already present in the vicinity of the welded portions and may be damaged by the sandblasting treatment thereby to reduce the adhesive strength. In order to remove the residual strain from the welded portions, moreover, the welded structures have to be subjected to a heat treatment, referred to as a Stress Releasing treatment ("SR-treatment"). Generally speaking, since the hard surface film to be formed by the spray-coating method has a small coefficient of thermal expansion, the coating surface film, as formed before the welding operation, may be separated or broken by thermal strain if a proper temperature is set at the SR-treatment. When the welded portions are coated with spray-coated surface films before the SR-treatment, on the other hand, other strains are caused in the surface films by the release, as effected by the SR-treatment, of the residual strain in the welded portions, so that a separation and breakage of the surface films become more likely to occur.